Undescended testis is the most common genitourinary anomaly with a prevalence at birth of 2–5 % and up to 30 % in preterm infants. Approximately 2:3 undescended testes will spontaneously descend within the first months of life, with a prevalence at the age of 1 year of 0.8 %. The testis may be located at any point along the descent route from the retroperitoneum to the external inguinal ring [18]. It can be ectopic, but in general it is found along the normal path of descent: approximately 70 % in the inguinal canal, 20 % in the prescrotal region just beyond the external inguinal ring, and the remaining in the abdomen. About 3–5 % of patients with undescended testis have congenital absence of the testis. It is conceivable that in most cases, the testis was originally present during fetal life and then regressed.

The major complications of cryptorchism are malignant degeneration, infertility, torsion, and bowel incarceration because of an associated indirect inguinal hernia. Tumors are far more common in an undescended testis than in a normally positioned testis.

Imaging is indicated to search for the location of the cryptorchid testis to evaluate its size and to seek for parenchymal lesions. The cryptorchid testis is usually smaller relative to the normally located testis because of testicular dystrophy and degeneration [19].

Ultrasonography can reliably detect testicles located in the inguinal canal or between the external inguinal ring and the scrotal neck. Detecting the testicle in intra-abdominal locations is often difficult or even impossible. MR imaging allows detecting the testis not only in the inguinal canal but also in the abdomen. High abdominal testes, however, may be difficult to identify, and a recent meta-analysis of an American pediatric group shows a low sensitivity [20]. DWI performed at high b-values (800–1,000 s/mm²) improves visualization of the undescended testis which appears hyperintense in most of cases [13] (Fig. 9.2). Severely atrophic testes, however, may show low signal intensity on diffusion-weighted imaging [21] (Fig. 9.3).

Congenital hydrocele occurs when there is incomplete closure of the processus vaginalis. It is very common in newborn but is encountered in less than 1 % of adults. The patent processus vaginalis permits flow of peritoneal fluid into the scrotum. Indirect inguinal hernias can be associated. There are two subtypes of congenital hydrocele: the encysted type, also called spermatic cord cyst, with no communication with the peritoneum and with the tunica vaginalis, and the funicular type, also called funiculocele, which communicates with the peritoneum at the internal ring but does not surround the testis.

Either ultrasonography or MR imaging can be used to evaluate congenital hydrocele. On ultrasound, it appears as an anechoic fluid collection surrounding the anterolateral aspects of the testis extending to the inguinal canal or as a well-demarcated anechoic mass, associated with either a patent or a closed internal ring, separated from the testis and epididymis which are displaced inferiorly. On MR imaging hydrocele is homogeneous and hypointense on T1-weighted images and hyperintense on T2-weighted images, characteristic of fluid. Occasionally, septations are identified, although prominent septations should suggest a hematocele or pyocele. Patency of the processus vaginalis with entry of peritoneal fluid into the scrotal sac is best demonstrated in heavily T2-weighted images.

Failure of the normal posterior anchoring of the epididymis and testis is called bell-clapper deformity because it leaves the testis free to swing and rotate within the tunica vaginalis of the scrotum like the clapper inside of a bell. It is a common precondition for testicular torsion, with a 12 % incidence in an autopsy series.

Bell-clapper deformity itself is hard to be detected by imaging. A suggestive feature is the abnormal direction of the long axis of the testis on the axial plane, in the right-to-left or anterior-posterior direction with the epididymis right upon the testis (Fig. 9.4). The deformity is more often recognized in heavily T2-weighted images when hydrocele is present and the tunica vaginalis completely encircles the epididymis, distal spermatic cord, and testis rather than attaching to the posterolateral aspect of the testis.

Polyorchidism is a rare congenital abnormality where more than two testes are present which may have separate or common spermatic cords and epididymis. Although three testes is the most common form, as many as five have been reported. In approximately 75 % of cases, the supernumerary testes are intrascrotal. Of the remaining cases, 20 % of the supernumerary testes are inguinal and 5 % retroperitoneal. On ultrasonography the accessory testis presents with the same echogenicity, echotexture, and vascular pattern as the normal testes. On MR imaging it has the same signal characteristics as the normal testes: intermediate signal intensity on T1-weighted images, high signal intensity on T2-weighted images, and hyperintensity on DWI (Fig. 9.5). Thanks to its panoramicity and improved tissue characterization, MR imaging is helpful in making a definitive diagnosis in case of equivocal ultrasound findings or extrascrotal testes [22].

Testicular adrenal rests derive from the hyperplasia of aberrant adrenal cortical tissue that descends during prenatal life with the testes or ovary. In approximately 8 % of patients with congenital adrenal hyperplasia, the elevated levels of serum adrenocorticotropic hormone stimulates these rests to grow. They are bilateral in more than 80 % of cases, usually presenting as palpable masses inside of or close to the mediastinum testis.

Testicular adrenal rests present on ultrasound as unilateral or bilateral lobulated masses with variable echogenicity located within or adjacent to the mediastinum testis. Acoustic shadowing may be due to fibrotic changes or calcifications. Lesions are isointense to the surrounding normal tissue of the testis on T1-weighted images and hypointense to the surrounding normal tissue of the testis on T2-weighted images with and without fat suppression. Strong enhancement is demonstrated after gadolinium administration.

The normal appendages are typically seen only when hydrocele is present [25–27]. They usually display intermediate signal intensity on T2-weighted images. In appendiceal torsion, a twisted and swollen appendage is usually seen as a hyperintense oval structure on T2-weighted images lacking intralesional enhancement with ring-like perilesional enhancement on dynamic subtraction contrast-enhanced images [2, 28].

MR imaging has been claimed as a problem-solving technique in doubtful cases of segmental testicular infarction [29, 30]. On T1-weighted MR images, infarcts may appear isointense to the surrounding normal testicular parenchyma, occasionally with foci of high signal intensity due to hemorrhage.

On T2-weighted images, the signal intensity is variable. The lesion is avascular after gadolinium contrast administration (Fig. 9.7), often with an enhancing perilesional rim in subacute infarction [29, 31]. Although segmental testicular infarction presents with characteristic MR features, this investigation is not commonly performed in our clinical practice, as similar information is provided by contrast-enhanced ultrasonography which is quicker and with a lower cost [32].

Tubular ectasia of the rete testis is a benign nonneoplastic condition thought to result from partial or complete obliteration of the efferent ducts. It is more common in men over the age of 55 years, often bilateral, and frequently associated with spermatoceles.

Using modern ultrasonographic equipment, a tubular ectasia of the rete testis is characterized in virtually all patients; in case of doubt, however, MR can show the dilatation of multiple small tubules of the rete testis appearing hyperintense on T2-weighted and heavily T2-weighted images. After gadolinium administration no internal enhancement is seen [16].

The classic ultrasonographic appearance of testicular epidermoid cyst is the “onion ring” pattern, consisting in alternating rings of low and high echogenicity, which represent layers of keratinized squamous epithelium. This appearance is virtually pathognomonic, but epidermoid cysts with an atypical appearance may be difficult to differentiate from tumors. On MR imaging an epidermoid cyst typically shows on T2-weighted images alternating zones of high- and low signal intensity [1, 34, 35] and signal restriction on DWI [11]. This appearance, however, is not constant (Fig. 9.8). Signal intensity on T1-weighted images is variable. No matter the imaging features, epidermoid cysts do not demonstrate internal flow on Doppler interrogation nor enhancement after gadolinium administration [16].

Spontaneous intratesticular hematoma is a rare entity with few reports in the literature, mimicking a neoplasm on grayscale ultrasonography. Diagnosis of a nonneoplastic condition, however, is suggested as the lesion lacks vascularization on color Doppler interrogation and contrast-enhanced ultrasonography [36, 37]. MR imaging confirms the diagnosis showing a lesion with the characteristic signal changes of blood. No enhancement is seen after gadolinium contrast administration (Fig. 9.9). Follow-up is recommended to ascertain lesion size reduction and signal intensity changes over time of the blood content.